1. Field of the Invention
The present invention relates to direct current linked, alternating current inverter drives for induction motors and machines having similar load characteristics.
2. Description of the Prior Art
In the past, alternating current induction motor drive circuits were generally either constant current drives, wherein the inverter output section was provided with a relatively constant direct current, or constant voltage drives wherein the inverter output section was provided with a relatively constant direct current voltage. Adjustable speed voltage-fed drives typically were either variable voltage, variable frequency square wave inverter drives or more recently pulsed width modulation inverter drives.
Pulsed width modulation drives included a constant link voltage input to a pulse modulation circuit which synthesized a variable voltage, variable frequency motor drive output.
A typical, square wave, variable voltage, variable frequency inverter drive is illustrated in FIG. 1. The inverter drive illustrated in FIG. 1 does not include the commutation circuitry required to force commutate the output silicon controlled rectifiers or thyristors. In these type of six-step inverter drives, each of the inverter legs was enabled for 180.degree. to provide an output line-to-line voltage generally similar to the six-step wave form illustrated schematically in FIG. 1.
In these square wave inverter type motor drives, the speed of the motor was controlled by varying the inverter commutation frequency. In order to maintain adequate torque capability it was necessary to also vary the DC link voltage to maintain a constant ratio of voltage to frequency. Hence, these types of drives were referred to as variable voltage, variable frequency, inverter drives.
Inverter drives typically used gate controlled, switched circuit elements such as thyristors or silicon controlled rectifiers, which were enabled by providing a gate bias voltage but which also required external forced commutation circuitry to provide reverse bias voltage to disable the switch. Alternatively, some inverter drives used gate controlled switching elements such as transistors, gate turn-off thyristors or insulated gate transistors. In circuits with these type of switching elements, current flow through the switch was interrupted by applying, or removing, base-to-emitter bias voltage. In either case, the switching element was used to interrupt current flow into the induction motor which inherently produced rapid voltage changes across the switching element, i.e., a high dV/dT.
Since most economically available switching elements could not tolerate these rapid voltage changes, typical inverter drives also included some capacitive buffering or snubbering to protect the switching element. Snubbering circuits, however, produced losses proportional to the amount of protection afforded. In the conventional snubbering circuit illustrated in FIG. 2, when the switch was opened, the voltage was controlled as current flowed readily through the diode into the capacitor. When the switch was closed, the capacitor discharged slowly through the resistor. Accordingly, each switching cycle produced losses on the order of one-half CV.sup.2. To reduce losses, it was thus necessary to keep the snubbering capacitance as small as possible, and prior efforts at improving switching efficiency were directed to reducing switching times and improving the voltage tolerance of switching elements to minimize snubbering losses. Additionally, conventional inverter drives required free-wheeling, rectifiers or diodes in anti-parallel with the inverter switching elements to recirculate the inductive current from the motor after switching. These free-wheeling diodes prevented reverse voltage on the output, thereby requiring reverse current flow for negative power, i.e., generating, which in turn required reverse thyristors in the drive input section to reverse power flow into the A.C. supply line. High voltage rate or high dV/dT switching was thus inherently complex and expensive.